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Toolpack: Configuring Fax Relay

Applies to version(s): v2.7

This is a list of fax Options. To see this list, select Profiles in the navigation menu. Edit the configuration to access the Fax/Modem Relay options.

Options Description
Enable Fax Relay This option is the global option telling if faxes (Passthrough or T38) will be relayed or not.
Detection Mode This option contains one of the following mode: Standard, Use relay mode on any tone or Always Relay.

Standard: Fax tones and T.38 re-invites will trigger fax relay mode. Modem tones and passthrough re-invites will trigger passthrough mode, though fax detection will still continue.
Use relay mode on any tone: Both fax and modem tones will immediately cause switch to the chosen relay mode (without going to passthrough first for modem tones).
Always relay: Calls are directly invited using the specified relay mode, without waiting for any fax/modem tone. This improves performance when the calls are know to always be fax or modem (e.g. when connecting with a fax server).

Relay Mode This option is to choose which mode of fax will be used. This option contains one of the following mode: T.38 or Passthrough.

T.38: This relay mode is used for faxes modulated in IP.
Passthrough: Fax passthrough encodes fax traffic with in a G.711 (ulaw or alaw) voice codec and sends it across the VoIP network as a voice call. In this mode, echo cancellers are disabled, there's a fixed jitter buffer and there's no Voice Activity Detection.

Modem vs. Fax distinction timeout This option is the time we wait after we got the CED, to get V21 flags.

This is to avoid to send a SIP re-invite in passthrough and few ms after, when we realize it's a faxes, send another SIP re-invite in T38. Since CED is both for modem and faxes, we have the choice to wait a few ms before taking the "final" decision. The best choice depends on the setup. In general, 0ms will work fine. If the endpoint is T.38, then the option "Always Relay" will override the "Modem vs. Fax distinction timeout". Otherwise, 500ms will save a re-invite, but takes longer to switch to passthrough if the final result is modem.

Fax tones detection duration This option is the maximum time (in seconds) to listen to FAX tones. After that delay, FAX tones are ignored. This avoids false FAX detection later in the call when a sound in an audio conversation is close to a FAX tone frequency. Default time 30 seconds. Use 0 seconds for always trying to detect fax tones.
Enable early Fax/Modem tone detection This option is used to detect longer tones (like CED) faster. The reason is some tones are "long" (few seconds). The drawback is that is has more chance to detect a false positive, since it is a single tone and appears frequently in a conversation. We suggest to keep this turned off to avoid these false positives.

Note: V.21 Flags will be detect as soon as they comes. These flags are fax specific.

Enable switching to FAX relay upon CNG tone This option allows to switch to fax relay as soon as we detect a CNG tone. We recommend to leave this option checked to switch to fax faster.
Use V.34 This option is to support V.34 faxes. By using that option, faxes speed can transfer upto 33.6 Kbps.

If not checked, faxes will only supports V.17 faxes (transfer speed upto 14.4 Kbps).

Max bitrate This option is used to limit the maximum bitrate of faxes. By default, faxes will try to train at maximum bitrate. If a Failure to Train (FTT) happens, the bitrate will be reduced again and again. This option is used when you want to specify the maximum bitrate.

By using that option, faxes transmission will take less time, since no FFT will be negociated.

Fill bit removal This option is used to remove inserted fill bits in Phase C for non-ECM data.

We suggest to leave this option unchecked.

TCF transfer mode Transfer training in T.38 or generate locally the Training Confirmation.

We suggest to leave this option at Transferred (default option).

Redundancy Count This option is to send multiple control frames, not FAX page data.
Transmission level (-1dBm unit) This option is to adjust the transmission level to the TDM network. Valid options are between 0 and 15 inclusive.
Detection type This option is used for different types of fax passthrough. When receiving an SIP invite, Our
Codec This option specify which codec will be used for fax passthrough. By nature, fax passthrough MUST be a non-compressed codec. So you can choose PCMU or PCMA.
Jitter Buffer Depth This option is to specify what will be the value of the fixed Jitter Buffer.

References